Handle loading .storybook/babel.config.js (#6633)

[LukeAskew]

#6633

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[shilman]

Great fix! Did you test it?

@ndelangen are there any good places to drop in tests for this?

[ndelangen]

@LukeAskew Would you be able to see what we can do to fix the failing cli test?
https://circleci.com/gh/storybooks/storybook/121204

[LukeAskew]

Sure, but I could use a little guidance.

1. These same cli tests fail locally when I run at the HEAD of next. I think this is because the babel configs in these fixtures don't include preset-react. I am unsure of why these pass on Circle.
2. What is the intended behavior of test cases like this? I would expect this config to be used instead of the default and I would expect this to fail because it lacks the necessary presets.
3. How can I link my changes in core to the cli test? It appears that the fixtures pull directly from npm. It is unclear how I can test my changes across the repo.

[igor-dv]

AFAIR, The similar fix was already considered some time ago. One problem is that it might be a breaking change. SB could start requiring babel configs from root, that are existing in the project and where not required before.

Also some discussion was here - #2582

[LukeAskew]

Updated the code to just handle .storybook/babel.config.js in addition to .babelrc. This allows folks to piggyback on the root config via:

module.exports = require('../babel.config');